The invention concerns a tool for logging while drilling. More specifically, the invention concerns a tool for logging a formation using pulsed nuclear magnetic resonance (NMR) techniques while drilling a borehole into the formation.
Several pulsed NMR tools have been designed for borehole deployment via wireline and are described in U.S. Pat. Nos. 4,350,955, issued September 1982 to Jackson et al.; 4,629,986, issued December 1986 to Clow et al.; 4,710,713, issued December 1987 to Strikman; 4,717,876, issued January 1988 to Masi et al.; 4,717,877, issued January 1988 to Taicher et al.; 4,717,878, issued January 1988 to Taicher et al.; 4,714,881, issued January 1987 to Givens; 5,023,551, issued June 1991 to Kleinberg et al.; 5,055,787, issued October 1991 to Kleinberg et al.; and 5,055,788, issued October 1991 to Kleinberg et al. All of these patents describe NMR tools which employ permanent magnets to polarize hydrogen nuclei, and RF antennas to excite and detect nuclear magnetic resonance to determine porosity, free fluid ratio, or permeability of a formation, for example. The tools described in U.S. Pat. Nos. 4,717,877; 5,055,787; and 5,055,788 have been successfully tested in boreholes.
Several logging while drilling (LWD) and measuring while drilling (MWD) tools have been designed for formation evaluation while drilling and drill string characterization while drilling, respectively. Logging or measuring instruments are placed in drill collars up to 100 ft behind the drill bit. An MWD tool is described in U.S. Pat. Nos. 3,777,560 to Guignard and 4,479,564 to Tanguy. An LWD tool is described in U.S. Pat. Nos. 4,899,112 to Clark et al. and 4,949,045 to Clark et al. Typically, these tools use electromagnetic techniques in evaluating resistivity of a formation while drilling.
U.S. Pat. No. 5,280,243 to Miller et al. describes a nuclear magnetic resonance (NMR) logging while drilling tool. According to this patent, the outer surface of a magnet is covered with an electrically insulative material. An RF antenna is mounted on the coated outer surface of the magnet. The RF antenna generates a magnetic dipole in a primary direction, perpendicular to the conductive drill collar surface, and the RF antenna generates an image magnetic dipole in the opposite direction, perpendicular to the drill collar surface. With the RF antenna electromagnetically isolated from the collar, the image magnetic dipoles in the opposite direction are smaller than the magnetic dipole in the primary direction, and a net RF magnetic dipole is generated for the NMR measurements. If electrically insulative coating is removed from the outer surface of the magnet, the magnetic dipole in the primary direction will have a magnitude substantially equal to the image magnetic dipole in the opposite direction. The magnetic dipole and the opposing image magnetic dipole will cancel each other, thus, disabling the NMR measurement.